In one known digital circuit, as disclosed in, e.g., JP-A-2001-327101, a backup circuit constituted by a backup capacitor is disposed between a power supply terminal and the digital circuit. When a voltage is supplied from the power supply terminal, the backup capacitor is charged, and when no voltage is supplied upon power cutoff, a voltage is supplied to the digital circuit with electrical charges that have been charged in the backup capacitor, thereby holding information stored in the storage circuit.
Then, to prevent the voltage charged in the backup capacitor from being supplied to the exterior from the power supply terminal, it is known to dispose a diode between the power supply terminal and the backup capacitor. An anode of the diode is connected to the power supply terminal, and a cathode of the diode is connected to one terminal of the backup capacitor. A current is thereby prevented from flowing backward from the backup capacitor toward the power supply terminal when the voltage supplied from the power supply terminal drops.
In the case using the diode, however, some special process, such as the SOI process, is required because the standard CMOS process cannot be used to constitute the diode and the digital circuit on the same LSI chip. Accordingly, when the digital circuit is constituted by using the standard CMOS process, a problem arises in that the diode must be formed as an external device. Another problem with the use of the diode is that the diode causes a voltage drop even during the normal operation.
On the other hand, it is also known to use, instead of a diode, a MOS transistor driven by an oscillator and a charge pump. In this known related art, the oscillator is driven by a voltage supplied from a power supply terminal, and the charge pump is driven by an output of the oscillator. Further, an anode of the MOS transistor is connected to the power supply terminal, and a cathode of the MOS transistor is connected to one terminal of the backup capacitor. An output of the charge pump is supplied to a gate of the MOS transistor. When the voltage supplied from the power supply terminal drops, the oscillator is stopped to reduce the voltage applied from the charge pump to the gate of the MOS transistor, whereby the MOS transistor is turned off to prevent a current from flowing backward from the backup capacitor toward the power supply terminal. Since a backup circuit having such an arrangement can be fabricated by the standard CMOS process, the backup circuit can be integrated on the same semiconductor chip as the digital circuit, and a voltage drop hardly occurs.